Soon after I posted this, I moved the Final Exam "out of" the Final Exam category and delted the "Final Exam" category. This whole thing behaves like a tree. The course is at the top, then there's a second level which might contain activities or categories. Now I have two categories ("Quizzes" and "Assignments") and a Final Exam activity on the same level. Since the categories and the activity are on the same level, they're all given weights in the weighted mean calculation. I gave it a weight of 0.4 and everything's working fine.